Flywheel power storage devices, and the various elements needed for their implementation, have been set forth in the prior art, having various forms and combinations. Numerous recent flywheel patents describe devices intended to serve as electromechanical batteries, which store kinetic energy in a spinning flywheel and generate electric power from it when needed. A safe, cost-competitive, practical and care-free flywheel power storage and regeneration system can provide a very attractive alternative to electrochemical batteries used in UPS (Uninterruptible Power Systems). Electrochemical batteries subjected to frequent deep-discharge have short service lifetimes, low reliability, need for costly and frequent maintenance, temperature limits that may compromise applicability, performance and reliability degradation while in use, and poor energy recovery efficiency. UPS is mostly used as on-site backup for conventional utility power distributed extensively to stationary sites connected to power grids.
In addition to storing and regenerating power onboard a spacecraft, especially an orbital satellite, practical electromechanical batteries, in system combinations, can also provide inertial attitude control for such a spacecraft. Orbital satellites are launched for global communications, radio, television, mapping, weather prediction, and myriad useful purposes.
My present invention includes improvements and enhancements, over teaching set forth in my U.S. Pat. No. 6,566,775. That invention mainly teaches a minimal-loss flywheel battery. It teaches means for achieving virtually zero xe2x80x9cidling lossxe2x80x9d (an electromechanical battery property that may be compared to electrochemical battery xe2x80x9ctrickle chargexe2x80x9d) while its magnetically levitated rotor spins at high speeds, with configurations that avoid magnetic cycling of magnetic materials, and that block and buck eddy currents in stator windings. That patent also teaches motor/generator means for ultra-high electromechanical power conversion efficiencies and nearly zero power loss while coasting at all speeds, and systems that can have virtually unlimited service life without need for maintenance. It also teaches power interface electronics, that exchange current with its DC (direct current) power buss and its motor/generator. And it teaches magnetic levitation means that require virtually zero steady-state power.
A high-speed radial-field regenerative motor, having virtually zero idling losses, is set forth there. It serves as a motor/generator, that alternately stores and regenerates power as needed. Its integral rotor assembly is configured to accommodate high spin speed without disintegrating from centrifugal forces, and to sustain virtually zero idling losses in a vacuum environment. Its power interface electronics controls current between the motor stator and a DC power buss, and is responsive to a variety of command signals. So the power electronics of a number of systems can be connected in parallel, to a DC power buss.
The present invention provides improvements over my U.S. Pat. No. 6,566,775. Both include a radial-field embodiment with non-contacting magnetic levitation, of a regenerative brushless DC motor. It is a flywheel version of the motor taught in my U.S. Pat. No. 4,520,300 for a coreless axial-field ultra-efficient regenerative servo including its electronic power control interface.
The flywheel assembly described in my U.S. Pat. No. 6,566,775 clearly is susceptible to an earthquake that might subject its external support structure to a free-fall. Only gravitational force acts to oppose axial magnetic attraction of its axial magnetic support. So subjecting the system to free-fall would cause the axial servo of its magnetic support to lose control.
Also, during a severe earthquake, the bi-directional current of its axial electromagnet coil may reach high levels at a polarity that induces reverse magnetic field on the concentric axial magnet. That may weaken the magnet by partially demagnetizing it.
Accordingly, an objective of this present invention is to remove those limitations, without increasing overall system losses, complexity, and production cost, or necessitating external devices such as shock absorbing mounts that would only mitigate such external disturbances.
Also, gravitational force is essential to proper operation of the flywheel battery axial and radial servos, described in U.S. Pat. No. 6,566,775. That would preclude use aboard a spacecraft. Accordingly, another objective of the present invention is to provide a xe2x80x9cweightlessxe2x80x9d environment embodiment especially suitable for use on orbital satellites, where flywheel system combinations can provide power storage and regeneration, plus attitude control.
A flywheel system with permanent magnets that support up to 90% of rotor weight is described in U.S. Pat. No. 6,262,505 by Hockney et al. That invention includes a cooperative electromagnet, to stabilize its otherwise unstable magnetic axial support. It also adds nominally 10% to total lift forces. No electromagnet current reversal means are described. So clearly, various external factors, such as an earthquake, or a temperature change that increases the magnet""s strength, which might reduce the electromagnet current to zero, would result in loss of axial position control. Also, that invention includes radial journal bearings with shock absorbing properties and clearances selected to limit off-center radial excursions. They would be subject to wear, that would reduce service life and contaminate a crucially needed vacuum.
Other configurations are described in U.S. Pat. Nos. 5,627,419 by Miller; 5,760,510 by Nomura et al; 5,777,414 by Conrad; 5,319,844 by Huang et al; and 5,844,339 by Schroeder et al. Yet other configurations are described in U.S. Pat. Nos. 5,705,902 by Merritt et al; 5,044,944 and 5,311,092 by Fisher; 5,107,151 and 5,677,605 by Cambier et al; 5,670,838 by Everton; also 5,708,312 and 5,767,595 plus 5,770,909 by Rosen et al.
These flywheel systems are representative of prior art that does not quantitatively address their idling losses. Without continual power input, their energy would be typically dissipated in less than an hour, due to high hysteresis and eddy losses. This energy loss, without supplying output power, is far worse than self-discharge exhibited by most electrochemical batteries.
None of these configurations, nor any other prior art, include the minimal-idling-loss means of the motor/generator, power interface electronics, and magnetic support elements, described in U.S. Pat. No. 6,566,775 and improvements plus enhancements thereto of the present invention.
Magnetic levitation without electronic servo loops is described in U.S. Pat. Nos. 5,495,221 and 5,783,885 plus 5,847,480 and 5,861,690 plus 5,883,499 by Post. Magnetic bearings, for use in flywheel batteries, that employ hysteresis and eddy effects, for moving mechanical devices, to adjust physical positions of magnetic materials for axial stability, confront serious stability and reliability problems. Axial lift-off by repulsion forces between fixed conductors and a spinning Halbach magnet array, may result in rotor lift and stabilization at speeds above those needing mechanical bearings for rotor support. But this method confronts high idling losses and high stray magnetic fields, from eddy currents required for both lifting and stabilizing their rotor assembly. Mechanical bearing wear and lubrication would also be troublesome. And they would confront formidable stability problems, because there is so little chance to optimize dynamic behavior. Conversely, the electronic servo loops controlling electromagnet actuators described herein, which comprise key elements of the present invention, are readily optimized.
Vacuum loss in some prior art would necessitate relatively frequent maintenance to keep windage loss at acceptably low levels. In turn, need for a vacuum enclosure that can be opened for frequent maintenance, further degrades the interior vacuum soon after re-closing an enclosure needing flexible seals. They are prone to leaking and outgassing. This problem is so pervasive, that some enclosures have a permanently connected vacuum pump. High temperatures cause mechanical bearing lubricants to boil and some composite fiber flywheel resins to outgas into a relatively small enclosed and evacuated space. In most prior art, the enclosed space has been small, to minimize size and weight of the enclosure, which has thick walls designed to contain a possible exploding flywheel rim. A small enclosure space, with high internal temperatures and materials that outgas, cannot reliably maintain a vacuum.
Laminated core stator motors have ample inductance for pulse-width modulation. But they are very lossy at PWM (Pulse-Width-Modulation) frequencies, and have considerable stator core losses at commutation frequencies. So they have low power conversion efficiency due to high PWM losses, plus high idling losses due to magnetic cycling incurred by the stator core while its permanent-magnet rotor spins (typically with a non-magnetic band around it for high speeds).
A type of motor/generator, known in the art as coreless (because its stator windings are not in slots of salient pole cores), has also been used in some prior art flywheel systems. They incur considerable eddy current losses in their stator windings, which has mistakenly been attributed to skin effect. Those with stepwise commutation also incur rotor hysteresis and eddy loss, when converting power. Rotor heat does not have a high thermal conductivity path to the enclosure, in systems having contactless magnetic bearings, so high rotor temperatures may be incurred. That type of motor/generator is substantially different from those taught in my U.S. Pat. Nos. 4,520,300 and 6,566,775. Moreover, further improvement to the power interface electronics of that motor/generator, to decrease PWM circuit losses while in motor drive mode, is set forth herein.
In some prior art, idling loss has been largely due to friction in mechanical bearings, and to motor/generators and magnetic bearings that magnetically cycle iron as the rotor spins, causing substantial hysteresis and eddy losses. Some prior art also includes many combinations of magnetic bearings that are stabilized and assisted by mechanical bearings of various types. Some use a motor/generator having standard mechanical bearings, coupled to a flywheel by materials having radial compliance to minimize vibration stresses on the mechanical bearings.
Mechanical bearings of some prior art would incur serious heating and wear, running in vacuum at sustained high speed. Very high operating temperatures of critical parts, have been caused by high localized heat generation compounded by low heat transfer, further compounded by lubrication loss accelerated by lubricant boil-off in vacuum. These conditions have resulted in early mechanical bearing wear, their subsequent deterioration, and their high failure rates. Their friction also causes high idling losses, and resulting high self-discharge rates.
Typical prior art motor/generators, used in flywheel assemblies, incur a substantial part of their loss in core laminations subjected to relatively high-frequency PWM current control. Also, idling loss due to iron cores and superconductors, that are magnetically cycled by Halbach array alternated permanent-magnet spinning rotors, causes high self-discharge rates. With such high power conversion and idling loss, excessive heat is generated within the evacuated flywheel enclosure. This heat can cause a variety of failure modes. It also can cause excessive maintenance requirements, which prevent practical and safe installation, of flywheel batteries intended for stationary on-site terrestrial use. Clearly, such maintenance on space satellites is not at all practical, and is dangerous to astronauts who would need to perform it.
U.S. Pat. Nos. 4,961,551 and 5,441,222 plus 6,288,670 by Rosen et al, describe flywheels used in combinations, on space satellites, to control satellite angular orientation. Some are used as devices called reaction wheels. They control angular orientation along axes parallel to the wheel spin axes, from bi-directional reaction torques caused by accelerating or decelerating rotor spin.
Other flywheels are used in devices called control moment gyros. With these devices, torque applied orthogonal to the rotor spin axis results in a proportional angular precession rate, along a mutually perpendicular axis. But their high idling losses preclude their use for power storage and regeneration. And they do not have radial electromagnets that can apply torque for achieving the precession rates needed to control satellite attitude. So they need external torquers and gimbals.
None of these flywheels provide power storage and regeneration, which is mainly provided by electrochemical batteries onboard orbital satellites. Moreover, their mechanical rotor and gimbal bearings incur troublesome lubricant loss, wear-out, and consequent system failures.
Rechargeable electrochemical batteries are commonly used for storing on-site electric power. All types require frequent maintenance, may fail without warning, and deteriorate over time. Their lifetimes are limited to less than ten yearsxe2x80x94far shorter if subjected to repeated frequent deep charge/discharge cycles or not promptly recharged after supplying power. That is typical at off-grid sites. These battery drawbacks have been a major obstacle to on-site solar and wind power installations. To provide power on demand, such installations require power storage that is subjected to daily and highly variable charge/discharge cycles. Standard test analysis, using failure prediction methods widely accepted by the engineering community, indicate that a practical flywheel system can operate under the same demanding service conditions, with a system mean-time-between-failure exceeding 50 years.
After satellite launch, battery maintenance and replacement are not practical. Benefits of the present invention would be profound, for storing and regenerating power on orbital satellites especially their ability to also provide inertial attitude control, from the same flywheel systems providing UPS. It could provide far higher reliability, than present electrochemical battery UPS plus attitude control by jet thrusters or reaction wheels. Jet thrusters have serious fuel limits, and their valves may stick, causing erratic action and system failure.
Prior art magnetic bearings have also been described for onboard flywheels that operate in a xe2x80x9cweightlessxe2x80x9d space environment, where mechanical bearing lubricants boil off, leading to early mechanical bearing wear-out and failure. The magnetic bearings include various configurations of permanent magnets, and electronic servos to adjust magnetic forces for axial alignment and stabilization. The flywheel systems they teach do not include means for achieving virtually zero idling losses; so they are not suitable for electric power storage and regeneration. They also do not include means for the magnetic bearings to apply torque orthogonal to their flywheel spin axis, to control attitude by precession around a mutually orthogonal axis.
The list of other flywheel and related element patents, included hereabove, represents a very small fraction of many patents, which describe many possible diverse flywheel configurations.
Flywheel power storage systems that are not subject to the aforementioned drawbacks and limitations would afford significant improvement to numerous useful applications. These include on-site UPS to sustain critical electric power functions (during grid power outages) at facilities commonly served by a central power grid, UPS plus power storage for distributed on-site solar and wind power systems, and satellite power storage plus attitude control.
General objectives of this invention are to provide more robust flywheel systems, for stationary installations, without the power losses, maintenance, possible earthquake damage, and malfunctions of prior art flywheel batteries, plus an alternate embodiment to provide power storage and attitude control for orbital satellites.
At high temperature, coercive force of permanent magnets is reduced. In most prior art, this has required critical adjustment of magnetic bearings, imposed higher loads on mechanical bearings or electromagnets that stabilize magnetic rotor support, and caused reduced torque vs. current of motor/generators, with reduced back-EMF vs. rotational speed.
Accordingly, it is a general objective of the present invention, to provide more robust magnetic levitation, for flywheel systems having automatically adjusted bus voltage and axial rotor assembly position, by electronic means described herein.
A primary and specific objective is to provide a flywheel system, which can withstand severe shock and vibration without damage, never subjects its magnets to reverse magnetic fields, and includes means to achieve high signal integrity. It includes a motor/generator with no magnetic hysteresis losses and virtually no eddy current losses; its rotor integral with magnetic bearings that need virtually no steady-state power to drive their axial and radial position stabilizing electromagnets. It will not incur magnetic hysteresis and eddy current losses, and can use low-cost magnets. All embodiments of the present invention include mechanical bearings as emergency backup only, not normally engaged when the rotor is spinning.
Another specific objective of this invention is to add improvements described herein so the system can tolerate earthquakes, including an alternate embodiment that can improve service life and reliability of orbital satellites. Both embodiments include high electromechanical power conversion efficiency means and minimal idling loss taught in my U.S. Pat. No. 6,566,775.
Another objective of this invention is to minimize internal losses and consequent heat in a more robust flywheel assembly which operates in vacuum, to minimize heat transfer needs.
Another objective is to provide a robust flywheel system, without need for mechanical backup bearing lubricants, to remove a cause of vacuum loss in stationary on-site terrestrial applications, prevent need for maintenance, and rely less on mechanical backup bearings.
Another objective is to maintain all electronic components and critical regions therein, at cool temperatures, with minimal temperature cycling. And since electronic power interface losses amount to a considerable part of total system power conversion losses, it is a specific objective to further minimize losses in the power interface H-bridge and related components.
Another specific objective is to provide shields against electro-magnetic-interference (EMI), for the rotor angle sensors and the axial and radial position sensors.
Another objective, consistent with and dependent upon the previous objectives, is to provide a robust flywheel system that does not require maintenance over a very long service lifetime; to reduce maintenance cost and especially to reduce need for access to the flywheel.
Another specific objective, consistent with and dependent upon the previous objectives, is to provide a power-up sequence controlled by electronics responsive to a manual or remote power-up command. This sequence would disengage mechanical rotor support, and then commence rotor spin-up, according to a power-up algorithm described herein.
Another specific objective, consistent with and dependent upon the previous objectives, is to provide a power-down sequence controlled by electronics responsive to a manual or remote user command, or upon sensing internal abnormal vibration. This sequence would inhibit further rotor speed increase, decelerate the rotor if possible by dumping electric power generated by its motor/generator into a discretionary load, and finally re-engage mechanical rotor support, when spin speed is reduced to a prescribed level that the mechanical support can handle.
Another objective of this invention is to reliably detect possible flywheel rim deterioration or seismic shaking, manifested by axial and radial servo activity. Signals from the axial and radial servos initiate the power-down sequence described above. Worst-case rotor spindle capture (that would occur if electronics also fails) will be provided by backup mechanical bearings. Such failsafe rotor capture means can minimize flywheel damage and prevent explosion from possible rim disintegration and possible concurrent electronic power-down control malfunction.
Yet another objective is to provide a flywheel system having virtually unlimited service life without maintenance for orbital space satellites, that can also provide inertial attitude control.
It is a specific objective, to provide magnetic bearings that are more robust; and power interface electronics that are more efficient, over those set forth in my U.S. Pat. No. 6,566,775.
And it is a specific objective, to provide system combinations of an embodiment that can store and regenerate electric power onboard orbital satellites, and also control satellite angular attitude.
Accordingly, flywheel systems and combinations, including their component elements, are herein described, for achieving these objectives, plus other advantages and enhancements. Key manufacturing processes, also needed to achieve these objectives, are also described.
A magnetically levitated rotor support, having a top cylindrical permanent-magnet and concentric axial electromagnet to provide force in an upward axial direction, plus a bottom axial electromagnet to provide force in a downward axial direction, is described herein. This configuration, along with gravity, also provides passive leveling and centering forces. It is intended to provide more robust magnetic bearings that better tolerate earthquakes and even free-fall without damage or interruption of system operation. Also, this configuration does not subject the permanent magnet to a reverse magnetomotive force, which may weaken it. So it is more robust over those set forth in my U.S. Pat. No. 6,566,775.
A second system embodiment intended for use on orbital satellites, includes a magnetically levitated rotor support, having a permanent magnet and axial electromagnet at one end, plus a like permanent magnet and axial electromagnet at an opposite end to provide force in an opposing axial direction. This combination also provides passive radial centering, from axial magnetics, at both ends of the rotor assembly. Spin axis precession torquing means are also set forth herein, to provide inertial attitude control for satellites. Mechanical backup bearings are included, to secure the rotor assembly on the last sequence of power-down, especially during satellite launch. An embodiment is described that also maintains closed high-permeability magnetic paths for both axial magnets, when the rotor assembly is mechanically secured.
My present invention includes the minimal-loss means described in my U.S. Pat. No. 6,566,775. It also sets forth more robust flywheel rotor assembly axial magnetic support means, for electric power storage and regeneration with minimal losses, that can tolerate external shock and vibration including free-fall, that the system might experience during severe earthquakes. It also sets forth axial magnetics configurations wherein magnetic fields from cooperative stabilizing electromagnets cannot weaken associated permanent magnets.
And it sets forth more efficient power interface means, by adding PWM turn-off delay to one switch of each diagonal pair in H-bridges, while they control the motor drive stator current.
The various subsystems and combinations improved by the present invention include:
(1) More robust integral magnetic bearings, with feedback control servos to stabilize rotor assembly centering and adjust rotor axial position for zero long-term bearing loss, and to withstand without damage external shaking that the system may incur during possible earthquakes, in a vertical-axis flywheel system for terrestrial power storage and regeneration.
(2) More efficient electronic power interface circuits, EMI suppression, plus a power-up and power-down sequence, controlled by algorithms described herein.
(3) Combinations of an alternate flywheel embodiment, for ultra-reliable electric power storage and regeneration plus inertial attitude control for space satellites, especially for orbital satellites powered by photovoltaic panels, including means for the magnetic bearings to apply precession control torques. They would provide a more reliable and longer life alternative to the prior art.
Improvements to the art will be apparent from the following description of the invention when considered in conjunction with the accompanying drawings wherein: